Combustible gas detecting apparatus



A ril 22, 1969 T. H. PALMER 3,440,017

I I COMBUSTIBLE GAS DETECTING APPARATUS Filed Feb. 28. 1966 H 1o METHANE Y 29 DETECTING-w DEVICE ;12

1 f 25 ON/OFF. 27 Y 19 TRANSISTOR v 15 AMPLIFIER 23 16 o 21 TEMPERATURE C 40 COMPENSATING METHANE DEVICE 13 WARNING ........26 LAMP FIG. 1

APPLICANT Thomas Henry Palmer United States Patent 3,440,017 COMBUSTIBLE GAS DETECTING APPARATUS Thomas Henry Palmer, Stafford, England, assignor to The English Electric Company Limited, London, England, a British company Continuation-in-part of application Ser. No. 173,761, Feb. 16, 1962. This application Feb. 28, 1966, Ser. No. 530,502 Claims priority, application Great Britain, Feb. 24, 1961, 6,842/61; Mar. 29, 1961, 11,485/61 Int. Cl. G01n 31/12 US. Cl. 23-254 5 Claims ABSTRACT OF THE DISCLOSURE A thermo-couple device for a methane detector comprises a thermo-couple comprising coaxial wires buttwelded and-to-end to form a hot junction, a closely-fitting glass sleeve enclosing the thermocouple element, and a heating coil encircling the glass sleeve and arranged symmetrically relative to the hot junction. The device may be enveloped in a catalytic material to render it responsive to the presence of methane.

This application is a continuation-in-part application of Palmer, Ser. No. 173,761, filed Feb. 16, 1962, now Patent No. 3,251,654 issued May 17, 1966.

This invention relates to apparatus for detecting the presence of relatively small quantities of a combustible gas, such as methane, in an atmosphere, and in particular to gas responsive and ambient temperature compensating elements for incorporation in such apparatus.

In present day industrial undertakings where the presence of combustible gases, such as methane, constitutes a particular fire and explosion hazard precautions must be taken to protect life and property by continuously monitoring the local atmosphere so as to detect when the concentration of a particular combustible gas in that atmosphere reaches a predetermined high danger level. This is particularly the case in coal mine working where methane may be found to be present in continuously varying concentrations, andin respect of which legislation in some countries requires the withdrawal of men and the cutting off of electrical supplies when the methane (or fire damp) concentration rises above a predetermined high value.

To conform with the legislation, for example of the of the United Kingdom, detecting means must be carried by specified personnel whenever proceeding to an underground working. Such detecting means must be readily portable, must have sufficient capacity to adequately last out the duration of a stay underground, and must moreover be entirely reliable in use and be able to withstand the rigours of being portable in underground mining conditions of operation.

The history of the development of methane detector is a long and arduous one extending over the last fifty or more years, and so far as the present applicant is aware no readily portable detector has been possible until recent times, when the progress of the various allied technologies has made it within mans grasp to produce a methane detector which goes some substantial way to meeting all of the requirements.

Portability in such a detector brings withit many requirements, for example,

(a) Lightness of weight;

(b) Compactness,

(0) Minimal power requirement to effect operation over the desired period;

(d) A conservation of the energy source when no Patented Apr. 22, 1969 warning of a hazard need be given, so as to avoid excessive size in the source; and

(e) Above all robustness and an ability to give reliable operation under the operating conditions to be experienced.

All of these requirements bear on the design and construction of the combustible gas responsive element on whose ability to indicate the presence of combustible gas the whole detecting apparatus depends for its successful operation. These requirements also hear on the design and construction of the element often provided in such apparatus for enabling compensation to be afforded for variations in the atmosphere ambient temperature.

It is known in the .prior art to use as a combustible gas responsive element a mass carrying a catalytic material which induces combustion of the combustible gas within the confines of an appropriate flame trap, and to employ some thermo-electric means for detecting an in crease in temperature of the mass brought about by the catalytic combustion around that mass. In some prior art constructions this thermo-electric means has included a resistance wire associated with the said mass and connected in an arm of an electric bridge detecting circuit. With such an arrangement an increase in the resistance of this resistance wire with rise in temperature of the said mass effects an unbalance of the bridge detecting circuit, and the unbalance is sensed by a sensitive volt-meter.

In other prior art constructions it has been proposed to use as a thermo-electric means a thermo-couple device for producing a thermoelectric E.M.F. dependent on the temperature of the mass, and a volt-meter has been provided for detecting the magnitude of this thermoelectric The present invention is directed to the provision of an advantageous construction of such a thermo-couple device for use in a combustible gas detecting apparatus.

According to the present invention a thermo-couple device for use in a combustible gas detecting apparatus includes (a) a thermo-couple element comprising two linear conductors of dissimilar metals disposed co-axially end to end and having their adjacent end fusion-welded together to form a hot junction of the thermo-couple element, the hot junction having a small volume and a diameter not greatly exceeding that of the two conductors, (b) enclosing the hot junction and adjacent parts of the therrno-couple conductors in a closely-fitting manner a thin-walled glass sleeve positioned symmetrically about the hot junction, (c) encircling the glass sleeve an electrical heating coil positioned symmetrically about the hot junction and fitting closely around the sleeve, and ((1) means for maintaining the electrical heating coil in position in relation to the hot junction and glass sleeve.

According to another feature of the present invention a combustible gas responsive device for use in a combustible gas detecting apparatus includes a thermo-couple device as set out above, in which device the glass sleeve and the heating coil are enveloped by a catalytic material for inducing about them combustion of a combustible gas and thereby a heating of the thermo-couple hot junction.

According to another feature of the present invention such a combustible gas detecting apparatus may include in addition to such a combustible gas responsive device a further 'thermo-couple device as set out above, the heating coil thereof being electrically connected in series with the heating coil of the combustible gas responsive device, and the thermo-couple element thereof being connected in series opposition with the thermo-couple element of the combustible gas responsive device.

According to another feature of the present invention such a combustible gas detecting apparatus includes in each thermo-electric device a base member carrying four electrically independent through connection and support posts, and in each such thermo-couple device the free ends of the thermo-couple conductors are connected to two of the said connection and support posts of the associated base member so that the thermo-couple element extends between those posts, and the ends of the associated heating coil are connected to the two other connection and support posts respectively.

According to yet another feature of the present invention such a combustible gas detecting apparatus includes for each such thermo-couple device a cup-shaped porous sintered metal cap secured in position on the associated base member so as to enclose the thermocouple device, the associated connection and support posts being of such lengths as to support the thermocouple element centrally within the enclosure formed by the associated base member and porous sintered metal cap.

One thermo-couple device according to the present invention for use in a methane detecting apparatus will now be described with reference to the accompanying drawing in which:

FIG. 1 shows diagrammatically the principal electric circuit connections of a combustible gas detecting apparatus in which the thermo-couple device may be used;

FIG. 2 shows a sectional elevation of the thermocouple device, revealing inter-alia the constructional details of a thermo-couple element used in the device; and

FIG. 3 shows a sectional elevation of a combustible gas responsive unit incorporating the thermo-couple device, this figure showing the arrangement of the thermocouple device in relation to associated parts in the unit.

Referring now to the FIG. 1 the combustible gas detecting apparatus includes two thermo-couples 10, 11 each including a hot junction 12, 13 and two constituent wires 14-17 of dissimilar metals. These thermo-couples are connected electrically in series opposition with one another so that an appearing at the ends of the wires 14 and 17 represents the difference of the thermoelectric E.M.F.s of the two thermo-couples, and hence the difference in temperature of the two hot junctions 12 and 13.

The net thermo-electric developed by the thermo-couples is applied to a sensitive millivolt meter 18 calibrated in terms of percentage concentration of methane, and through a potential divider 19 to the input circuit of an ON/ OFF transistor type amplifier 20 having positive feedback action and having connected to an output circuit thereof a methane warning lamp 21.

The thermocouples have associated with their respective hot junctions electric heating coils 22, 23 which are connected in series for supply from a supply battery 24. The manner of association of each heating coil and its associated thermo-couple hot junction will be described in detail later in relation to the other figures of the drawing, but suffice it to say here that each heater coil is electrically insulated from its associated thermocouple by a sleeve 25, 26 about which the heating coil is disposed, and within which is arranged the thermo-couple. In FIG. 1 the heating coils are not shown in true association with the thermo-couples simply for the sake of clarity.

Also in association with the heating coil 22 and the thermo-couple hot junction 12 is a mass of catalytic material (not shown in FIG. 1) which in the presence of methane and air in an associated enclosing flame trap 27 induces therein combustion of the methane and a consequent rise in the temperature of the hot junction 12 in dependence upon the concentration of methane in the atmosphere.

In contradistinction the heating coil 23 and associated hot junction 13 have no associated catalytic materials so that the hot junction 13 produces a thermo-electric E.M.F. which is solely dependent on the ambient temperature of the atmosphere which has diffused into a 4 flame trap 28 which encloses the heating coil 23 and hot junction 13.

The heating coil 22 is heated by the battery current in order to raise the temperature of the catalytic material to a more effective operating value. The heating coil 23 is likewise heated by the battery current so as to similarly raise the temperature of the other thermocouple hot junction 13.

The purpose of providing the heating coil 23 and hot junction 13 is to enable the net supplied to the millivolt meter 18 and the potential divider 19 to be independent of changes in the temperature of the hot junction 12 which result from variations in the ambient temperature of the atmosphere diffusing into the flame trap 27. This desired compensation for variations in ambient temperature is in fact achieved by the arrangement described since both thermo-couple hot junctions are similarly affected by changes in ambient temperature and because the thermo-electric E.M.F.s due to ambient temperature oppose one another and hence are cancelled out. Thus the calibration of the apparatus is rendered independent of the ambient temperature of the atmosphere being tested. This feature is truly important since a false indication of high methane concentration resulting from a variation in ambient temperature would result in a costly shutting down of production in a mine working.

The circuit arrangement so far described has advantages which have been extolled in relation to prior art arrangements in the documents of record in respect of the co-pending patent application No. 173,761, now Patent No. 3,251,654 (to which the readers attention is directed) of which this present application is a continuation-in-part.

The thermo-couple hot junction 12 and associated heating coil 22, insulating sleeve 25, and catalytic material will hereafter be referred to collectively as the gas responsive element 29, whilst the thermo-couple hot junction 13 and associated heating coil 23 and insulating sleeve will hereafter be referred to collectively as the atmosphere ambient temperature compensating element 30, or more simply as the compensating element 30.

The physical construction of the gas responsive element 29 is shown in FIG. 2, and its disposition and manner of support in the gas responsive unit is shown in FIG. 3.

In FIG. 2 it will be observed that the thermo-couple wires 14, 15 are disposed co-axially end to end, having been fusion-welded together to form the hot junction 12. This junction has a girth diameter which is not substantially greater than the diameter of the thermo-couple wires 14, 15, and has a volume which is relatively small for a thermocouple, the thermal inertia of this hot junction being consequently low and enabling the hot junction to quickly acquire the temperature of its surroundings.

The thermo-couple wires 14, 15 are enclosed in the electrically-insulating, closely-fitting, thin walled glass sleeve 25 which extends equally on either side of the hot junction 12, this sleeve having been formed about the thermo-couple wires and junction by heating a thin walled glass tube after first sliding it into position symmetrically disposed with respect to the hot junction.

Disposed co-axially and concentrically with the insulated thermo-couple wires is the platinum heating coil 22, which after being wound independently on a mandrel is slipped over the glass sleeve into the position shown (disposed symmetrically with respect to the hot junction 12) and is thereafter secured in this position by the deposition around the heating coil of a composition 31 which includes a suitable catalytic material for inducing the desired combustion of methane.

In operation the battery current is arranged to maintain the temperature of the heating coil at about 450 C., and in the presence of methane in the operating range of concentrations the heating coil temperature may rise to approximately 650 C., so that this coil will of its own not have a very great working mechanical strength, and so needs adequate support against vibration.

In the FIG. 3 it will be seen that the gas detecting element 29 is disposed substantially centrally in the flame trap 27, being so disposed as to ensure so far as possible that convection currents set up within the flame trap by the combustion and the consequent heating of the gas detecting element 29 do not vary in a substantial manner with variation of the angular disposition of the gas responsive unit as a whole.

The flame trap comprises a non-porous insulating base member 32 (for example of glass) carrying an outer flange 33 on to which is sealed a porous sintered metal thimble or cap 34. Secured in the base member 32 and protruding on both sides thereof are four stout electrical connection and support posts 35 to 38.

Between the turned over upper ends of the posts 35 and 36 extends the gas responsive element 29', the latter being supported firmly in position by virtue of fusionwelded connections made between these said ends of the posts 35 and 3 6 and the respective free ends of the thermocouple wires 14 and 15. The ends of the heating coil 22 are connected by fusion welding to the respective upper ends of two other posts 37 and 38.

Electrical connection to the thermo-couple and to the heating coil is made through connections to be'applied to the lower ends of the posts 35-38 which extend outside the flame trap 27.

The flame trap thimble 33 is preferably as described in the allowed Patent No. 3,237,181 (T. H. Palmer) to which the readers attention is hereby directed for particulars of its construction and merits.

The temperature compensating elements 30 is generally similar in construction to the gas responsive element 29, and is secured in its associated flame trap 28 in a manner similar to that in respect of the gas responsive element.

It will be appreciated however that in the case of the temperature compensating element 30 the heating coil 23 is not enveloped in a composition (such as 31) which includes a catalytic material. It is preferred that this heating coil is not secured in any way on the glass sleeve,

but rather is left as a loose fit around the sleeve, this coil being adequately supported by its ends and support posts 37, 38.

In relation to the known prior art as evidenced in the prosecution of the aforementioned co-pending patent application No. 173,761 the present gas responsive and temperature compensating elements have substantial advantages over the prior art devices. For example, in relation to the prior art thermo-couple constructions as evidenced by G. F. Taylor at page 797 of the August 1920 issue of the Journal of Industrial and Engineering Chemistry the present construction provides a very reliable and robust hot junction in which the constituent wires are firmly united by fusion-welding as opposed to being merely twisted together; in which the hot junction has only a very small thermal mass and can thus respond to small temperature changes in its surroundings without any substantial delay; which can be made in large quantities to practical limits of reproducibility (that is the construction enables a repeatable high accuracy to be obtained); in which because of the end to end disposition of the thermo-couple wires the whole gas responsive element can be supported in a strong robust way by means of the relatively strong thermo-couple wires (strong in comparison with the relatively poor strength of the heating coil wires which operate at high temperature), the heating coil itself contributing little if any supporting effect to the rest of the element; and in which because of the end to end disposition of the thermo-couple wires a thermally symmetrical arrangement is possible in which equal amounts of heat are lost from the hot junction along the two thermo-couple wires and in which the hot junction is situated in the hottest part of the heating coil, thus making best use of the electrical energy supplied to heat that coil.

As compared with a twisted end thermo-couple with the constituent thermo-couple wires projecting from the same side of the hot junction mass no extra supporting means has to be provided such as would otherwise have to be provided for supporting the hot junction of the prior art thermo-couple. Such an extra support would be undesirable in that it reduces the thermal symmetry of the arrangement and provides an extra path for heat dissipation from the thermo-couple hot junction.

The low diameter of the hot junction of the elements according to the present invention is achieved by means of fusion-welding by bringing together in a sideways, i.e., co-axial motion the ends of the thermo-couple wires, the ensuing contact between the wire ends discharging through a spark a capacitor which has been previously charged to a predetermined level (this level being dependent upon the volume of the weld to be made).

With this low diameter of hot junction and the mere single thermo-couple wire which the heating coil has to encircle, the heating coil can have smaller physical dimensions as compared with a heating coil arranged to fit over a prior art twisted end thermo-couple, so that the dissipation of electrical energy in the heatingcoil to maintain the catalyst and thermo-couples at the operating temperature is considerably smaller than it would otherwise be. This has the effect of enabling the gas detecting apparatus to function over a longer period for a given size of power source, or otherwise a reduction in the size of the power source.

Furthermore, the use of the opposed wire thermocouple avoids the necessity to insulate from one another the two wires forming the thermo-couple. In the prior art thermo-couples the wires lie adjacent one another and hence need to be insulated from one another.

The use of the opposed wire thermo-couple of the present invention enables the proper location of the element in the flame trap without the provision on the actual gas responsive and temperature compensating elements of special supporting members which serve no function other than that of providing support. Such special supporting members would be necessary with the prior art thermo-couples since the type of support used in the present element is not possible with the twisted wire thermo-couples. In the present element the thermocouple wires and the heating coil ends are the only means by which the elements derive their support in relation to their flameproof enclosures.

What I claim as my invention and desire to secure by Letters Patent is:

1. In a combustible gas detecting apparatus a thermocouple device which includes (a) a thermo-couple element comprising two linear conductors of dissimilar metals disposed co-axially end to end and having their adjacent ends fusion-welded together without additional metal to form a hot junction of the thermo-couple element, the hot junction having a small volume and a diameter not substantially greater than the diameter of the thermo-couple conductors, (b) enclosing the hot junction and adjacent parts of the thermo-couple conductors in a closely-fitting manner a thin-walled glass sleeve positioned symmetrically about the hot junction, (c) encircling the glass sleeve an electrical heating coil positioned symmetrically about the hot junction and fitting closely around the sleeve, and (d) means for maintaining the electrical heating coil in position in relation to the hot junction and glass sleeve.

2. A combustible gas detecting apparatus according to claim 1, wherein the glass sleeve and the heating coil are enveloped by a catalytic material for inducing about them combustion of a combustible gas and thereby a heating of the thermo-couple hot junction, the aforesaid thermo-couple device and catalytic material together constituting a combustible gas responsive device.

3. A combustible gas detecting apparatus according to claim 2, including a second thermo-couple device which includes (a) a thermocouple element comprising two linear conductors of dissimilar metals disposed coaxially end-to-end and having their adjacent ends fusion-welded together to form a hot junction of the thermo-couple element, the hot junction having a small volume and a diameter not greatly exceeding that of the two conductors, (b) enclosing the hot junction and adjacent parts of the thermo-couple conductors in a closely-fitting manner a thin-walled glass sleeve positioned symmetrically about the hot junction, (c) encircling the glass sleeve an electrical heating coil positioned symmetrically about the hot junction and fitting closely around the sleeve, and ('d) means for maintaining the electrical heating coil in position in relation to the hot junction and glass sleeve, first electrical connection means electrically connecting the heating coil of the second thermo-couple device in series with the heating coil of the aforesaid combustible gas responsive device, and second electrical connection means electrically connecting the thermo-couple element of the second thermo-couple device in series opposition with the thermo-couple element of the aforesaid combustible gas responsive device.

4. A combustible gas detecting apparatus as claimed in claim 3, including for each thermo-couple device a base member carrying four electrically independent through connection and support posts; in which in each such thermo-couple device the free ends of the thermocouple conductors are connected to two of the said connection and support posts of the associated base member so that the thermo-couple element extends between those posts; and in which in each such thermo-couple device the ends of the associated heating coil are connected to the other two connection and support posts respectively.

5. A combustible gas detecting apparatus as claimed in claim 4, including for each such thermo-couple device a cup-shaped porous sintered metal cap secured in position on the associated base member so as to enclose the thermo-couple device, the associated connection and support posts being of such lengths as to support the thermocouple element centrally within the enclosure formed by the associated base member and porous sintered metal cup.

References Cited UNITED STATES PATENTS 2,234,128 3/1941 Miller 23-23 2,335,032 11/1943 Sullivan 23-23 2,583,930 1/1952 Cotton 23-255 2,768,069 10/1956 Thompson 23-255 FOREIGN PATENTS 333,498 3/1921 Germany. 924,353 2/1955 Germany.

OTHER REFERENCES Taylor, F., Ir.: Instrumentation 5, pp. 35 and 36 (1952). Copy in POSL. 23255E.

Taylor, G. F.: J. Ind. and Eng. Chem. 12, 797.

MORRIS O. WOLK, Primary Examiner.

R. M. REESE, Assistant Examiner.

US. Cl. X.R. 23-232 

